The use of microcapsules for both the slow or controlled and fast or quick release of liquid, solid and solids dissolved or suspended in solvent is well known in the chemical art, including the pharmaceutical, specialty chemical and agricultural industries. In agriculture, these release techniques have improved the efficiency of herbicides, insecticides, fumgicides, bactericides and fertilizers. Non-agricultural uses have included encapsulated dyes, inks, pharmaceuticals, flavoring agents and fragrances.
The material used in forming the wall of the microcapsule is typically taken from resin intermediates or monomers. The wall tends to be porous in nature, and may release the entrapped material to the surrounding medium at a slow or controlled rate by diffusion through the wall. The capsules may be alternatively designed so as to quickly release the material to the surrounding medium by modifying the cross-linkage in the wall. Further, the encapsulated material may be released in either a controlled or quick manner by means of a trigger mechanism built into the wall, wherein the trigger may be environmentally sensitive allowing quick breakdown of the wall under certain conditions. In addition to providing controlled or quick release, the walls also serve to facilitate the dispersion of water-immiscible liquids into water and water-containing media such as wet soil. Droplets encapsulated in this manner are particularly useful in agriculture.
Various processes for microencapsulating material have been previously developed. These processes can be divided into three broad categories—physical, phase separation and interfacial reaction methods. In the physical methods category, microcapsule wall material and core particles are physically brought together and the wall material flows around the core particle to form the microcapsule. In the phase separation category, microcapsules are formed by emulsifying or dispersing the core material in an immiscible continuous phase in which the wall material is dissolved and caused to physically separate from the continuous phase, such as by coacervation, and deposit around the core particles. In the interfacial reaction category, the core material is emulsified or dispersed in an immiscible continuous phase, and then an interfacial polymerization reaction is caused to take place at the surface of the core particles thereby forming microcapsules.
The above processes vary in utility. Physical methods, such as spray drying, spray chilling and humidized bed spray coating, have limited utility for the microencapsulation of products because of volatility losses and pollution control problems associated with evaporation of solvent or cooling, and because under most conditions not all of the product is encapsulated nor do all of the polymer particles contain product cores. Phase separation techniques suffer from process control and product loading limitations. It may be difficult to achieve reproducible phase separation conditions, and it may be difficult to ensure that the phase-separated polymer will preferentially wet the core droplets.
Interfacial polymerization reaction methods have proven to be the most suitable processes for use in the agricultural industry for the microencapsulation of pesticides. There are various types of interfacial reaction techniques. In one type of interfacial condensation polymerization microencapsulation process, monomers from the oil and aqueous phases respectively are brought together at the oil/water interface where they react by condensation to form the microcapsule wall (“two phase polymerisation”). In general such reactions involve the condensation of an isocyanate moiety on one monomer with a second moiety such as an amine on a second monomer.
In another type of polymerization reaction, the in situ interfacial condensation polymerization reaction, all of the wall-forming monomers or pre-polymers are contained in one phase (the oil phase or the aqueous phase as the case may be). In one process the oil is dispersed into a continuous or aqueous phase solution comprising water and a surface-active agent. The organic phase is dispersed as discrete droplets throughout the aqueous phase by means of emulsification, with an interface between the discrete organic phase droplets and the surrounding continuous aqueous phase solution being formed. In situ condensation of the wall-forming materials and curing of the polymers at the organic-aqueous phase interface may be initiated by heating the emulsion to a temperature between of about 20° C. to about 85° C. and optionally adjusting the pH. The heating occurs for a sufficient period of time to allow substantial completion of in situ condensation of the monomers or pre-polymers to convert the organic droplets to capsules consisting of solid permeable polymer shells enclosing the organic core materials.
Many such in situ condensations involve isocyanate moieties. For example one type of microcapsule prepared by in situ condensation and found in the art, as exemplified in U.S. Pat. No. 4,285,720 is a polyurea microcapsule which involves the use of at least one polyisocyanate such as polymethylene polyphenyleneisocyanate (PMPPI) and/or tolylene diisocyanate (TDI) as the wall-forming material. In the creation of polyurea microcapsules, the wall-forming reaction is initiated by heating the emulsion to an elevated temperature at which point the isocyanate groups are hydrolyzed at the interface to form amines, which in turn react with unhydrolyzed isocyanate groups to form the polyurea microcapsule wall.
Isocyanates may undergo many types of chemical transformations such as homopolymerisation, oligomerisation, cycloaddition, insertion and nucleophilic reactions as described in the text H. Ulrich, CHEMISTRY AND TECHNOLOGY OF ISOCYANATES, John Wiley & Sons, Chichester, United Kingdom (1996). In the context of microcapsule wall formation, nucleophilic reactions are the most important. Typical nucleophiles include carboxyl, thiol, active methylene, hydroxyl and amino groups.
The use of isocyanates in which the —NCO group is ‘masked’ is well known in isocyanate polymer chemistry. For example, the —NCO group may be reacted with various molecules (BH) to give blocked isocyanates (RNHCOB) as described in Wicks & Wicks, PROGRESS IN ORGANIC COATINGS, Vol. 36, pp. 148-72 (1999). The blocked isocyanates may be de-blocked by further reaction with nucleophiles:RNCO+BH→R—NH—CO—BR—NH—CO—B+NuH→R—NH—CO—Nu+BHWhile we do not exclude the use of blocked isocyanates in the present invention, that approach is not preferred as it normally requires relatively high (>100° C.) temperatures for the deblocking reaction, and as the blocking agents are released into the medium.
A further type of microcapsule prepared by in situ condensation which does not involve the reaction of isocyanate groups is exemplified in U.S. Pat. Nos. 4,956,129 and 5,332,584. These microcapsules, commonly termed “aminoplast” microcapsules, are prepared by the self-condensation of etherified urea-formaldehyde resins or prepolymers in which from about 50 to about 98% of the methylol groups have been etherified with a C4-C10 alcohol (preferably n-butanol). The prepolymer is added to or included in the organic phase of an oil/water emulsion. Self-condensation of the prepolymer takes place under the action of heat at low pH. To form the microcapsules, the temperature of the two-phase emulsion is raised to a value of from about 20° C. to about 90° C., preferably from about 40° C. to about 90° C., most preferably from about 40° C. to about 60° C. Depending on the system, the pH value may be adjusted to an appropriate level. For the purpose of this invention a pH of about 1.5 to 3 is appropriate.
Microcapsules produced by such in situ condensation have the benefits of high pesticide loading and low manufacturing costs, as well as a very efficient membrane and no reactive residue remaining in the aqueous phase.                Regardless of the type of process utilized, the final encapsulated products may be packaged and used in a number of forms. For instance, they may be used in the form of a suspension of microcapsules in a liquid such as water or another aqueous medium (generally termed a suspension concentrate). Alternatively they may be packaged and used as dried microcapsules (for instance, produced from suspensions of microcapsules in liquids by techniques such as by spray drying, flat plate drying, drum drying or other drying methods). In yet a third way, they may be combined into other solid formulations such as granules, tapes or tablets containing microcapsules. All of these types of formulations are generally used by adding them to a liquid medium (usually water) in equipment such as a spray tank for agricultural use. Liquid media, whether that packaged with the microcapsules in suspension concentrate form, or that used in a spray tank or other application equipment, often has various ingredients in addition to water, including wetters, dispersants, emulsifiers, protective colloids or colloid stabilizers and surface active agents or surfactants. The protective colloids are used in processes for the preparation of the microcapsules and serve to prevent agglomeration of the oil droplets prior to encapsulation, or of the capsules after wall formation, as well as aiding the re-dispersing of the capsules upon settling. The surfactants perform various functions depending upon the type of surfactant used. These include varying the permeability of the wall, aiding in dispersing the capsules, acting as a wetter, reducing or eliminating foaming, affecting the adhesiveness of the capsule to the surface to which it is applied, and so forth. Primarily, the surfactants act as free, non-bound emulsifiers in the preparation of the precursor emulsion. However, under certain conditions the protective colloids, surfactants and emulsifiers can become desorbed or otherwise separated (to varying degrees) from the microcapsules so that they do not continue to perform their functions as effectively.        